A. Field of the Invention
The field of the invention relates generally to lighting fixtures or luminaires, and more particularly to a luminaire that incorporates a flexible OLED (organic light emitting diode) assembly.
B. Description of Related Art
Organic electroluminescent devices, such as organic light emitting diodes (OLEDs), are widely used for display applications, and the use of such devices in general lighting applications is gaining acceptance. An OLED device includes one or more organic light emitting layers disposed between two electrodes (e.g., a cathode and an anode) formed on a substrate. An encapsulating cover is disposed over the cathode and functions to seal and protect the underlying layers. The OLED device may be “top-emitting,” wherein the produced light is emitted through the cover, or “bottom-emitting,” wherein the produced light is emitted through the substrate. The organic light emitting layers emit light upon application of a voltage across the anode and cathode, whereby electrons are directly injected into the organic layers from the cathode and holes are directly injected into the organic layers from the anode. The electrons and the holes travel through the organic layers until they recombine at a luminescent center. This recombination process results in the emission of photons. i.e., light.
A growing use for wide area (typically larger than 200 cm2) OLED devices is as a light source in lighting fixtures or “luminaires.” Conventional OLED devices are, however, generally flat, planar devices, which limits their usefulness in certain lighting applications. In this regard, efforts have been made to modify OLED devices for use in more conventional three-dimensional lighting fixtures. For example, U.S. Pat. No. 7,075,226 describes a lighting apparatus that includes a flexible OLED device that can be transported and stored in a flat configuration, and subsequently configured into a three-dimensional shape within a lighting fixture.
OLED devices are an efficient, high-brightness light source, but are not without certain inherent drawbacks, particularly when used as a luminaire light source. Many types of OLED devices include a plurality of individual OLED elements formed on a common substrate, which results in the individual OLED elements being visually distinguishable. This characteristic may not be desirable in certain luminaire applications. In addition, the light from an OLED device is particularly bright, and may be too harsh for certain types of luminaries. OLED devices may include differently colored OLED elements, which may be beneficial for advertising displays and the like, but may not be desirable in wide area lighting applications.
U.S. Pat. No. 6,776,496 describes a lighting apparatus that utilizes a flat, planar OLED device removably received in a lighting fixture. The '496 patent discourages the use of “bulky reflectors and diffusers” and suggests that the substrate or cover layer may be frosted to provide light diffusion. This arrangement requires additional processing of the OLED device and, once the device is made, provides no option as to the nature or diffusive properties of the emitted light.
Therefore, an ongoing need exists in the industry for an improved, more versatile OLED device that is particularly suitable as a light source in a wide variety of lighting fixtures and applications.
FIG. 1 is a top planar view of a conventional OLED device 10 that may be utilized in an OLED assembly in accordance with aspects of the invention. The OLED device 10 is illustrated as a generally flat, planar member having a width 31 and a length 33. It should be appreciated that the rectangular shape of the OLED device 10 in FIG. 1 is for illustrative purposes only, and that a suitable OLED device 10 may have any desired shape, size, or other configuration.
The OLED device 10 in FIG. 1 includes a plurality of individual OLED devices 14 configured on a suitable rigid or pliable substrate 12. The OLED elements 14 are disposed lengthwise across the substrate 12, and each OLED element defines a generally continuous, unbroken light region 26. The OLED elements 14 are separated by scribe lines or gaps 28 that are formed during the deposition process wherein the various material layers are deposited on the substrate 12, as is well known in the art. The OLED device 10 has an active light area 30 that is defined essentially by the surface area of the OLED elements 14, particularly the light regions 26. An OLED device 10 having an active light area 30 of greater than about 200 cm2 is generally considered as a “large” or “wide” area device, and is particularly well-suited as a light source that may be incorporated into any manner of luminaire or light fixture.
FIG. 2 is a cross-sectional view of a bottom-emitting OLED device 10 wherein light is emitted through the substrate layer 12, which is clear or translucent. The OLED device 10 could also be a top-emitting device wherein light is emitted through an opposite cover layer 24. A first electrode layer 18 is deposited on the flexible substrate 12, which may be designated as the anode layer for sake of reference. For a bottom-emitting device, the anode layer 18 is also transparent. The anode layer 18 generally comprises a material having a low work function value such that a relatively small voltage causes emission of electrons from the anode 18. The anode 18 may comprise, for example, indium tin oxide (ITO), tin oxide, nickel, or gold. The anode 18 may be formed by conventional deposition techniques, such as vapor deposition, sputtering, and so forth.
One or more layers of organic light emitting materials 20 are deposited on the anode 18. A variety of organic light emitting material layers are known for use in OLED devices. In the embodiment of FIG. 2, the organic light emitting layer 20 is a single layer, and may comprise, for example, a conjugated luminescent polymer, a hole-transporting polymer doped with electron transport molecules and a luminescent material, or an inert polymer doped with hole transporting molecules and a luminescent material. The organic light emitting layer 20 may also comprise an amorphous film of luminescent small organic molecules, which can be doped with other luminescent molecules. According to other embodiments of the invention, the organic light emitting layer 20 may include two or more sub-layers which carry out the functions of hole injection, hole transport, electron injection, electron transport, and luminescence.
A cathode layer 22 is deposited on the organic light emitting layer 20 by any suitable deposition technique. The cathode layer 22 may comprise, for example, calcium or a metal such as gold, indium, manganese, tin, lead, aluminum, silver, magnesium, or a magnesium/silver alloy. Alternatively, the anode can be made of two layers to enhance electron injection. Examples include a thin inner layer of LiF followed by a thicker outer layer of aluminum or silver, or a thin inner layer of calcium followed by a thicker outer layer of aluminum or silver.
FIG. 2 depicts the individual OLED elements 14 as defined by “cutting” through the anode 18 and light emitting layer 20 (for example in a laser scribing technique), as indicated by the scribe lines 28. The cathode 22 is then applied as a common electrode layer to the plurality of OLED elements 14, and is thus considered a “high work function” layer in that it must be capable of carrying current for all of the OLED elements 14.
A protective cover 24 is typically applied over the cathode layer 22 and forms a generally hermetic seal over the underlying layers. This cover may be formed from various suitable materials, including an oxide or nitride coated semiconducting or insulating polymer (e.g., polyethylene terephthalate, PEN, or other enforced transparent polymer), or a thin ceramic (e.g., silicon nitride, oxide, or combination of both). In a particular embodiment, the cover 24 may incorporate a thermally conductive layer, such as one or more layers of a metal or metal alloy, for example silver, aluminum, tin, copper, steel, and so forth. Alternatively, the cover 24 may be formed from a thermally conductive material, such as aluminum nitride.